1. Field of the Invention
The present invention relates to a method of automatically controlling a transmission power of a wireless communication apparatus, and also to a storage medium on which the method is stored.
2. Description of the Related Art
Most of wireless communication apparatuses transmit data in units of block, to a base station. In the case where data transmission has been successfully carried out, an acknowledge (ACK) signal is received from the base station. In the case where data transmission has nor been successfully carried out, a negative acknowledge (NACK) signal indicative of a retransmission request is issued from the base station. When the NACK acknowledge signal is received, the data is retransmitted to the base station.
A rechargeable battery is used as the power source of a wireless communication apparatus. As power consumption of the wireless communication apparatus is larger, the life of the battery is shorter. In order to prolong the life of a battery, therefore, power consumption of a wireless communication apparatus must be controlled so as to suppress waste power consumption.
An example of a method of reducing power consumption of a wireless communication apparatus is disclosed in Japanese Unexamined Patent Publication JP-A 10-13338 (1998). In the method, a predetermined number m of successive transmission successes is preset, and the transmission power is adjusted in the following manner. In the case where data transmission is successfully carried out m times, the transmission power is reduced by a predetermined amount. In the case where data transmission ended in failure one time, the transmission power is increased by the predetermined amount.
FIG. 8 is a flowchart showing a conventional method of automatically controlling a transmission power of a wireless communication apparatus which is disclosed in JP-A 10-13338. When the wireless communication apparatus is powered on at step S301, the apparatus is set to the initial state so that a transmission success counter which counts the number of transmission successes is initialized to set x=0 (S302).
Thereafter, the apparatus transmits data to a base station, and then waits for an answer signal, i.e., an ACK signal or a NACK signal replied from the base station (S303). at step S304, it is determined whether an ACK signal is received or not.
In the case where at step S304 it is judged that the ACK signal has been received, the transmission success counter is incremented by 1 (S305). At step S306, it is determined whether the transmission success number x is equal to the predetermined number m or not. In the case where it is judged that the transmission success number x is not equal to the predetermined number m, the process returns to step S303. In the case where the ACK signal is then again received at step S304, the transmission success counter is incremented by 1 (S305).
It is again determined at step S306 whether the ACK signal is received the predetermined number (=m) of times or not. In the case where it is judged that the signal has been received m times, the transmission power is reduced by the predetermined amount (S308), and the process then returns to the state of waiting for the answer signal (S303).
In the case where at step S304 it is judged that the NACK signal has been received, or it is judged that the ACK signal is not received, the transmission success counter is reset at step S309 to 0, and then the transmission power is immediately increased by the predetermined amount (S310). Thereafter, the process returns to step S303.
In the above conventional art, the transmission power is controlled as described above. Namely, communication is carried out while reducing power consumption, so that the transmission power at the present time has the optimum value, whereby the life of a battery is prolonged. The optimum value of the transmission power means the minimum value of the transmission power at which reception by a base station is enabled at the present time.
Next, the level transition of the transmission power in a wireless communication apparatus of the conventional art will be described. In the conventional method of automatically controlling a transmission power, the transmission power at the end of the previous communication is selected as that at the start of the present communication. FIG. 9 is a graph showing the level transition of the transmission power in a wireless communication apparatus of the conventional art in the case where the transmission power in the previous communication is higher than the optimum value of the present communication. FIG. 9 shows the level transition of the transmission power in the case where, each time when the ACK signal is successively received the predetermined number (in this example, m=3) of times, the transmission power is reduced by a predetermined amount. In this example, after reduction of the transmission power from the initial value is carried out three times, the NACK signal is received, and the transmission power is then immediately increased by the predetermined amount.
FIG. 10 is a graph showing the level transition of the transmission power in the case where the transmission power in the previous communication is lower than the optimum value of the present communication. In the example of FIG. 10, after operations of receiving the NACK signal and then immediately increasing the transmission power by the predetermined amount are carried out three times, the ACK signal is received and data transmission at the power is carried out the predetermined number (in this example, m=3) of times. In the case where these data transmissions are successfully carried out and the ACK signal is successively received m (=3) times, the transmission power is reduced by the predetermined amount. In the case where the NACK signal is then again received, the transmission power is immediately increased. Thereafter, the control is repeated in which, when the ACK signal is successively received three times, the transmission power is reduced by the predetermined amount, and, when the NACK signal is again received, the transmission power is immediately increased.
In the method of automatically controlling a transmission power of the conventional art, the transmission power is increased only by the predetermined amount as described with reference to FIG. 10. When the transmission power of the previous transmission is lower than the present optimum value by several steps, the transmission power must be increased in the several steps. Therefore, the method has such a problem that data transmission successively fails before the transmission power reaches the optimum value.
Even in a state where data transmission is carried out at the optimum transmission power, in the case where data transmission succeeds the predetermined number of times, as shown in FIG. 10, the transmission power is reduced by the predetermined amount, and therefore the possibility of data transmission failure is increased. In the case where data transmission fails, data retransmission must be carried out at an increased power. Even though the transmission power is reduced with the intention of suppressing power consumption, the power, contrarily, is wastefully used for the data retransmission.